INFLUENCE OF TEMPERATURE ON STRONTIUM ADSORPTION ON HEMATITE

Abstract

Adsorption on iron oxide surfaces controls, to a considerable extent, the mobility of trace elements in sediments and soils. The isotope 90Sr, which is produced in the nuclear reaction of uranium fission, is among the most hazardous radioactive substances. Using a combined method of acid-base potentiometric titration and strontium concentration measurement in solution, a model was developed for the calculation of equilibria in the heterophase system hematite (≡ FeOH)-H+-Sr2+. The influence of pH and sorbate /sorbent concentration ratio (Sr2+/≡FeOH) on the character of adsorption was studied at temperatures of 25, 50, and 75°C. The experimental data were interpreted using the extended constant capacitance model for the double layer. Thermodynamic data were retrieved for the description of acid-base equilibria and reaction of Sr complex formation on the hematite surface at 25, 50, and 75°C. The experimental data appeared to be most consistent with a model including two surface complexes with stoichiometry that can be interpreted as formation of inner-sphere monodentate complexes according to the following reactions: ≡FeOH + Sr2+ ↔ ≡FeOHSr 2+ and ≡FeOH + Sr2+ + H2O ↔ ≡FeOSrOH + 2H+. The possibility of formation of outer-sphere strontium complexes is suggested by the values of ΔH and ΔS obtained from the formation constants of the surface complexes. The enthalpy of Sr adsorption is 48 and 147 kJ/mol for ≡FeOHSr2+ and ≡FeOSrOH, respectively. The value of ΔH calculated for the ≡FeOSrOH complex is much higher because its formation always includes the co-adsorption of two hydroxyl ions. Moreover, the entropy of formation of the complex ≡FeOHSr2+ (196 kJ K -1 mol-1) is similar to the entropy of Sr 2+ hydration in aqueous solution (205 kJ K-1 mol-1), which suggests almost complete dehydration of Sr 2+. An increase in temperature and the endothermic character of the reactions of surface complex formation promote strontium adsorption. Strontium adsorption and immobilization in natural environments are hardly possible under normal temperatures but could be significant at elevated temperatures, for instance, in the sites of underground disposal of radioactive waste.